It's pretty impossible to tell if it will get bigger as we can't tell what it is . If it is a cyst, there are many different kinds that can only be diagnosed accurately via biopsy. I do understand your worry though. If it was one of my dogs, I'd make an appointment as soon as I possibly could (within a week but only if there wasn't any change in it's appearance nor the dogs' behavior).

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"Education is a progressive discovery of our own ignorance." -Will Durant